While Hakyung will do anything to save his family, he's not an opportunist. Proof is the part when the granpa gave him money because for him, Myungsoo's already paying him enough. It was already said that Yoonsung's family did their best to support his mom's medication. They did their part until the end. It's his mother's body that can't take it. This is also the reason why Gunwoo wanted to work in their hospital. You see they're not that bad for Hakyung to just pay no heed of his deal with the family. He's doing his fair share of dealings.
His guilt roots from something else.
i think it's rooted in his sexuality since he felt it was better to betray yoonsung than to expose his gender and be hated by yoonsung but in the end we won't know until he tells us.
i was really waiting for whatever hakyung had done to him to be life altering betrayal. but he just chose to save his mom over revealing the truth so if i were him i wouldn't even feel guilty for what i had done. i would have made the same choice for my family. i mean i probably would've just too yoonsung the truth and asked for his help but hakyung's guilt is too much. his mom didn't even survive. yoonsung will forgive him.